1. Field of the Invention
The present invention relates to a microphone and, more particularly, to a grounding structure of an output transformer thereof.
2. Related Background of the Invention
Since a signal level of signal output of a microphone is low, a balance signal is used for output. For a conversion into a balance signal output, an output transformer with a center tap is used in most cases. Since a smaller output transformer of a microphone is preferable if possible, high-permeability material is used as a core material. Examples of high-permeability material include a permalloy. A core board made of high-permeability material such as the permalloy is heat-treated so that magnetic properties are in best conditions, and then the core boards are stacked to produce a core and the core is wire-wound to assemble a transformer. Magnetic properties of the core of a transformer made of high-permeability material, typically permalloy, deteriorate when heated. Also, the core, if deformed or impacted, has a propensity that magnetic properties thereof deteriorate under an influence of an internal stress or the like.
If a core of an output transformer of a microphone is grounded while the output transformer is incorporated in the microphone, external noise can be made to escape to a ground and mixing of noise with a sound signal converted by the microphone can be prevented. It is true, however, that some low-priced microphones do not have a grounded core of an output transformer, and in such microphones, noise may be mixed with a sound signal under the influence of external noise.
An internal structure of a general microphone, particularly a structure of an output transformer portion will be described below with reference to FIGS. 3A to 3C and subsequent figures. In FIGS. 3A to 3C, the microphone has, roughly speaking, a microphone case 50, a microphone unit 10 incorporated in a front end (left end in FIGS. 3A to 3C) of the microphone case 50, a circuit board 30 disposed inside the microphone case 50, an output transformer 20 disposed adjacent to a rear end of the circuit board 30, and a connector 40 disposed by connecting to the rear end of the circuit board 30 in a rear end of the microphone case 50. A form of the microphone unit 10 is not specifically limited and the example shown is a condenser type microphone. The connector 40 is a standardized 3-pin type connector and has three pins 41.
The output transformer 20 is mounted on the circuit board 30 and an end of a primary coil and that of a secondary coil are electrically connected to a predetermined circuit pattern by soldering or the like. FIGS. 4A to 4C show an appearance of the output transformer 20, and FIG. 4A is a plan view, FIG. 4B is a front view, and FIG. 4C is a right side view. In FIG. 4A, FIG. 4B, and FIG. 4C, the output transformer 20 has a core 21 and a coil 22 wound around the core 21. The core 21 comprises, for example, a U-shaped core board and an I-shaped core board that is stacked on an open end of the U-shaped core board to connect the open end. By inverting an orientation of the open end of the U-shaped core board and a position of the I-shaped core board alternately, that is, by stacking while turning around by 180 degrees alternately on a plane parallel to a core surface, the core 21 is formed as a rectangular frame. The coil 22 consists of a primary coil and a secondary coil, and the output transformer 20 is constructed by winding each coil around a bobbin and assembling each bobbin in a way in which one bobbin is inserted into one side of the rectangular frame and the other bobbin into the opposite side. To make a signal output of the microphone a balance output, the secondary coil of the output transformer 20 is a coil with a center tap.
As described above, the core 21 of the output transformer 20 is grounded to avoid influences of external noise. The core 21 can be grounded by electrically connecting it to a grounding pattern of the circuit board 30. FIG. 5A, FIG. 5B, and FIG. 5C show an example thereof. This is an example in which a grounding pattern is formed on the circuit board 30 in a position opposite to and in contact with the core 21 of the output transformer 20 and the core 21 is soldered to the grounding pattern while the output transformer 20 is put on the circuit board 30 to fix the output transformer 20. In FIG. 5A, FIG. 5B, and FIG. 5C, reference numeral 26 denotes solder for the grounding.
However, like the example shown in FIG. 5A, FIG. 5B, and FIG. 5C, soldering of the core 21 of the output transformer 20 directly to the grounding pattern is not preferable. This is because heating of the core 21 during soldering causes a temperature to rise, leading to degradation of magnetic properties of the core 21.
Another conventional example in which the core 21 of the output transformer 20 is grounded is shown in FIG. 6. In this example, a thin metal plate 27 to which a lead wire 28 is soldered on one end thereof is used. The metal plate 27 is inserted between core boards constituting the core 21 of the output transformer 20 disposed and fixed at a predetermined position of the circuit board 30 so that electricity is conducted between the core 21 and the metal plate 27, and the lead wire 28 is soldered to a grounding pattern of the circuit board 30 to electrically connect the core 21 to the grounding pattern. Since, excluding this grounding structure, the structure is the same as that of the conventional example shown in FIG. 5A, FIG. 5B, and FIG. 5C, a description thereof is omitted.
Since, according to the conventional example shown in FIG. 6, the thin metal plate 27 is inserted between core boards, a stress (internal stress) is disadvantageously applied to the core boards constituting the core 21, leading to degradation of magnetic properties.
In still another conventional example of grounding the core 21 of the output transformer 20, the core 21 is tightened with a bare conductive wire when fixing the output transformer 20 to the circuit board 30 and the grounding occurs through the conductive wire. However, this grounding structure also has disadvantages that a stress is applied to the core boards by a tightening force of the conductive wire, and when an impact force is applied to a microphone, the impact force is also applied to the output transformer through the conductive wire to add mechanical damage, leading to change of magnetic properties.
Incidentally, as a conventional technology relating to the present invention, an output connector of a microphone equipped with a base made of an electrical insulator mounted at a rear end of a microphone grip of a condenser microphone, wherein a grounding pin and two signal pins on a hot side and a cold side are provided on the base by passing through it and the grounding pin is connected to the microphone grip via predetermined conducting means, characterized in that at least a top face and a circumferential surface of the base are covered with an electrostatic shield member that is non-conducting to each of the signal pins and conducting to the grounding pin is known (See Patent document 1).
Also, a microphone apparatus is known, wherein a sound collecting pore is provided in a cabinet of the microphone apparatus, a microphone is fixed in the microphone apparatus on a central axis of the sound collecting pore, a conductive net using a conductive material is provided between the sound collecting pore and a sound input surface of the microphone, the cabinet is blocked from the outside by filling the sound collecting pore with the conductive net, and the conductive net is grounded so that, when an object charged with static electricity approaches the sound collecting pore, static electricity discharged from the object is grounded through the conductive net (See Patent document 2).                [Patent document 1] Japanese Patent Application Laid-Open No. 2005-94575        [Patent document 2] Japanese Patent Application Laid-Open No. 2005-86231        
An invention described in Patent document 1 is related to a grounding structure between a microphone grip and a grounding pin of an output connector, and an invention described in Patent document 2 is related to a structure in which a sound collecting pore of a microphone is covered with a conductive net and the conductive net is grounded. No patent document that has disclosed an installation structure of an output transformer of a microphone like the present invention could be found.